Batch of changes that creates a PhantomNet portal branding.

Also includes some PhantomNet-specific restrictions (e.g. only
allows use of the main Utah Emulab testbed aggregate).
This excercise stretched the limits of what we can reasonably do
before introducing real per-testbed branding/policy mechanisms to
the php/web front-end. My changes ain't exactly pretty...
Please take care when adding any testbed-specific changes to the
code. There are three flavors now to consider in the logic.